The present invention relates to processes for treating tris(2,3-dibromo-1-propyl) phosphates, more simply called dibromopropyl phosphate herein, to render the impurities contained therein harmless for the various intended uses, and more particularly, the present invention relates to processes for neutralizing the crude dibromopropyl phosphate prepared by the reaction of phosphorus oxychloride with dibromopropanol.
Such dibromophosphates contain certain limited quantities of one or both of the following phosphoric acid esters: EQU (CH.sub.2 Br--CHBr--CH.sub.2 O).sub.2 PO(OH) EQU ch.sub.2 br--CHBr--CH.sub.2 O--PO(OH).sub.2
and possibly also traces of hydrochloric and/or hydrobromic acids, representing altogether a total acidity equivalent to a maximum of about 500 meq/kg. Under ideal conditions the reaction of phosphorus oxychloride with dibromopropanol proceeds according to the equation: EQU 3 CH.sub.2 Br--CHBr--CH.sub.2 OH + POCl.sub.3 .fwdarw. (CH.sub.2 Br--CHBr--CH.sub.2 O).sub.3 PO + 3 HCl (1)
This reaction can be catalyzed by metallic compounds such as magnesium, magnesium chloride, titanium tetrachloride, tetrabutyl titanate [Ti(OC.sub.4 H.sub.9).sub.4 ], lithium, lithium chloride, and aluminum chloride, as shown in French Pat. No. 1,198,196 (the principal patent and the first addition thereto) and British Pat. No. 1,098,637. Under the usual industrial conditions for carrying out the reaction, the hydrochloric acid by-product causes the cleavage of a certain number of ester linkages and the concomitant formation of phosphoric acid acid esters and of dibromochloropropane by reactions such as: EQU (CH.sub.2 Br--CHBr--CH.sub.2 O).sub.3 PO + HCl .fwdarw. (CH.sub.2 Br--CHBr--CH.sub.2 O).sub.2 PO(OH) + CH.sub.2 Br--CHBr--CH.sub.2 Cl (2) EQU (CH.sub.2 Br--CHBr--CH.sub.2 O).sub.2 PO(OH) + HCl .fwdarw. (CH.sub.2 Br--CHBr--CH.sub.2 O)PO(OH).sub.2 + CH.sub.2 Br--CHBr--CH.sub.2 Cl (2')
The presence of one or both of the phosphoric esters, (CH.sub.2 Br--CHBr--CH.sub.2 O).sub.2 PO(OH) and CH.sub.2 BR--CHBr--CH.sub.2 O--PO(OH).sub.2, in the dibromopropyl phosphate is undesirable for uses such as fireproofing of plastics, synthetic fibers, and paints.
It can generally be attempted to suppress these side reactions by facilitating the removal of the dissolved hydrogen chloride, for example, by conducting the reaction under vacuum or with a flow of inert gas, but it is impossible totally to obviate these side reactions. The answer, as proposed in U.S. Pat. No. 3,046,297, is to operate in the presence of a tertiary amine, but this is uneconomic because of the necessity of adding considerable amounts of diluent as well as the need for multiple and delicate washes to remove the amine chlorohydrate by-product.
In commercial practice, one must accordingly be content in most instances with removing the unwanted phosphoric esters by means of alkaline washes. However, operating in this manner involves the total loss of these acid esters and also creates problems because of the high viscosity of the dibromopropyl phosphates, which in turn leads to difficulties in decantation and mandates again the use of a water-immiscible diluent which must thereafter be separated or removed by distillation. This involves substantial complication of the process, loss of solvent, reduction of production, and it appreciably increases the manufacturing cost.
It was suggested in French Pat. No. 1,198,196 that the acid phosphoric acid esters in the product be converted into neutral dibromopropyl phosphates, instead of being removed. According to this patent, the acid phosphoric esters are converted to neutral esters by reaction with an epoxide according to the reaction: ##STR1## the dibromopropyl radical being represented here, as elsewhere in this disclosure, by R.
A second improvement, described in the first addition to the foregoing Patent, involves converting the terminal hydroxyl group formed in foregoing reaction (3) to an acyloxy group, for instance, by reaction with acetic anhydride: EQU (RO).sub.2 P(O)--OCHR.sup.1 --CHR.sup.2 --OH + Ac.sub.2 O .fwdarw. (RO).sub.2 P(O)--OCHR.sup.1 --CHR.sup.2 --OAc + AcOH
this last procedure does not provide satisfactory results in all instances, particularly when the phosphoric ester is destined to be used in contact with aqueous environments for prolonged times. This is because the mixed dibromopropyl and .beta.-acyloxyalkyl esters formed according to this process are less resistant to hydrolysis than the tris(dibromopropyl phosphate itself.
It is thus necessary to arrange for treatment of the dibromopropyl phosphate obtained by reaction of phosphorus oxychloride with dibromopropanol, leading to a transformation of the impurities to products as stable to hydrolysis as the dibromopropyl phosphate itself.